Polycrystalline diamond compact (PDC) drill bits are set with PDC cutters mounted to bit blades. Many methods for defining the setting patterns for such PDC cutters are known in the art. The goals to be achieved with respect to any PDC cutting pattern include: enhancing the force balancing of the drill bit; improving the cleaning of the bit face; evening out the wear of the cutters across the bit face; improving the durability of the bit; and achieving improved rates of penetration by more effectively attacking the rock to be drilled.
Two known setting patterns with respect to PDC cutters are: the “single set” method and the “plural set” method. In the single set method, each PDC cutter that is positioned across the face of the bit is given a unique radial position measured from the center axis of the bit outwards towards the gage. One commonly utilized technique for implementing a single set pattern is to define a spiral function originating at the bit axis and then place individual PDC cutters at points where the spiral function intersects each blade location. The spiral-blade intersection points will each be located at a distinct radial distance from the bit axis. With respect to a plural set pattern (also known as “redundant cutter” or “tracking cutter” pattern), PDC cutters as deployed in sets containing two or more cutters each, wherein the cutters of a given set are positioned at a same radial distance from the bit axis. Because of the reduced area near the center of the bit face, there may be fewer blades, and thus not every PDC cutter on the bit is assured to be a member of a set positioned at the same radius, but the majority of the included cutters do belong to a set. The typical plural set pattern distributes the cutters included in each set (at the same radius) across the bit face (for example, on opposite blades).
Single set PDC bits tend to drill faster for a given total cutter density than do plural set PDC bits. A weakness with PDC single set bits lies in the fact that if an individual cutter is damaged or lost then wear is accelerated on the cutters in proximate radial positions to the lost cutter. This can lead to premature failure of the drill bit. Plural set PDC bits are typically more durable than single set PDC bits, but are also known for lower rates of penetration.
A near constant theme in the prior art concerning plural set bits holds that at least one of the circumferentially trailing cutters of a given cutter set should be exposed less than the leading cutters. This is done in the hope that the bit will act as a light set low cutter density bit until the primary cutters are worn down and then act as a heavier set higher cutter density bit in deeper, harder drilling situations when the less exposed secondary cutters come into play. In practice, however, these plural set PDC bits have slow rates of penetration even when first deployed and have even slower rates of penetration when they become minimally worn.
Early examples of plural set bits include bits where the cutter sets were deployed symmetrically in circumferential placement (i.e., 180 degrees apart for sets with 2 cutters and 120 degrees apart for sets with 3 cutters). This type of bit could incorporate a uniform location on the bit profile for all of the cutters on the bit. More recent examples of plural set PDC bits tend to have decreased or offset location on the bit profile of the trailing cutters in the cutter sets or if the location on the bit profile is equal they tend to vary the overlap of adjacent cutter sets to create areas where the rock face will be ridged while drilling to limit lateral vibration of the bit. These more recent designs are characterized by having an even number of blades and tend to have the cutter sets deployed symmetrically in circumferential placement.
The setting of cutters on bits has been well documented in the patent art. Reference is made to Williamson U.S. Pat. Nos. 4,429,755 and 4,545,411, the disclosures of which are hereby incorporated by reference. Reference is also made to Keith and Mensa-Wilmot U.S. Pat. Nos. 5,238,075, 5,265,685, 5,549,171, 5,551,522, 5,582,261, 5,592,996, 5,607,024, 5,607,025, 5937,958, and 6,164,394, the disclosures of which are hereby incorporated by reference. Reference is also made to Cortes U.S. Pat. No. 3,696,875, the disclosure of which is hereby incorporated by reference. Reference is further made to McClain U.S. Application for Patent No. 2008/0179108, the disclosure of which is hereby incorporated by reference.